1. Summary of the Disclosure
The invention relates to a photonic crystal fiber, in particular single-mode fiber for the IR wavelength range, and a process for producing a microstructured fiber of this type in a drawing process.
2. Description of the Related Art
Glass fibers for conducting light, as have been described many times, for example in Lexikon der Optik, pages 213-214, usually comprise a combination of two materials having a different refractive index, a relatively highly refractive light-conducting core material which is sheathed in a material having a lower refractive index. In general, these are glass materials, with the glass material for the light-conducting core glass being different from the glass material of the sheathing glass having a lower refractive index. Such a structure enables light to be conducted along the axis of the fiber in the core by means of total reflection at the interface between core and sheath, without light exiting outward through the sheath. Such fibers are referred to as stepped index fibers.
In such a fiber, the core glass has to have a very high transparency for the desired wavelength of radiation to be conducted, so that the absorption losses in the fiber can be kept very low. For the transmission of CO2 laser radiation, it is therefore necessary to use a material which has a very high transmission in the laser wavelength range from, for example, 9 μm to 12 μm. The only known class of materials which is transparent enough in the wavelength range mentioned is the chalcogenides. Core-sheath fibers can be produced therefrom but these have a relatively high absorption of 5 dB/m, i.e. only 30% of the input radiation reaches the fiber outlet after a distance of 1 m.
However, such fibers can only be subjected to extremely low laser powers of a few watts since they display very high absorption for higher powers, leading to strong heating and thus destruction of the fibers. These fibers are therefore unsuitable for industrial use.
As an alternative to the above-described fibers, fused silica tubes can, in order to achieve improved transmission of CO2 laser radiation, be coated on the inside with silver in order to reflect the CO2 laser radiation and thus achieve transmission of the radiation in this glass tube over a certain distance of from one meter to a number of meters. Typical absorptions are in the range above 1 dB/m. An alternative possibility is to coat the inside of glass tubes with up to 40 interference layers consisting alternately of a polymer and a chalcogenide glass. A reflectivity such that the CO2 laser radiation is reflected at the inner wall and thus transmitted within such a fiber tube is achieved, for example, at a wavelength of 10 μm. Typical absorptions or dampings are in the range above 1 dB/m.
All these fiber tubes are not mode-maintaining, i.e. they are multimode fibers.
Both variants having a coated inner tube have the disadvantage that, due to the high absorption, only pulsed laser operation at a low laser power is possible since otherwise the fibers would heat up and be destroyed as a result of the absorption. Even at low powers, the life of such a fiber is only a few hours of operation. For this reason, such fibers are usually used only once, e.g. for medical applications in laser surgery. In addition, these fiber tubes have a very large internal and external diameter of up to 1 mm, which allows only a very large bending radius.
Another type of optical fibers are photonic crystal fibers, (PCF). In such glass fibers, the light is conducted not by means of the refractive index of different materials, for example different glasses, but by an effective refractive index difference within the material generated by means of a gas, in particular air. On the subject of such fibers, reference may be made, for example, to P. St. J. Russel, “Photonic Crystal Fibres”, Science 299, 358 - 362 (2003) and also P. St. J. Russel “Photonic Crystal Fibres”, J. Light Wave Technology, 24(12), 4729-4749 (2006), the disclosure content of which is fully incorporated by reference into the present patent application. The effective refractive index difference within the glass generated by means of gas, in particular air, is achieved by means of a hole structure arranged around the light-conducting core. The light-conducting core can be either a solid material or a gas, in particular air or a noble gas such as argon.
The effective refractive index difference in the PCF is achieved by means of a periodic hole structure arranged around the light-conducting core.
PCFs are usually made of fused silica. Here, suitable glass tubes are assembled to give a preform, with a tube in the middle being replaced by a rod of the same size which later forms the light-conducting core. Such a preform contains up to several hundred individual tubes and usually has a diameter of 50 mm. In a subsequent single-stage or multistage drawing process, the preform is drawn down to a 125 μm fiber, with the hole structure having to be made smaller to the same extent. As an alternative, the rod in the middle of the structure can be omitted when drawing the fiber, so that a hollow core fiber is formed. If the symmetry and accuracy of the microstructure in the fiber is good enough, this fiber, too, also conducts radiation as in a classical core-sheath fiber on the basis of the band gap effect, which will not be described in more detail here. Since no absorbing medium is present in the middle of these hollow core fibers, CO2 laser radiation should in principle be able to be conducted thereby. However, conventional structured hollow core fibers composed of fused silica are no longer transparent above a wavelength of 5 μm and the CO2 laser radiation has to interact with the microstructure within the hollow core in order to achieve the band gap effect. Since the CO2 laser radiation is absorbed when it impinges on the fused silica, there is no occurrence of a band gap effect but instead the fiber is heated to destruction.
CN 10 298 1212 A has disclosed PCFs which are composed of tellurium glasses and are transparent in the wavelength range from 3 μm to 5 μm. The PCFs described in CN 10 298 1212 A are described as single-mode fibers but CN 10 298 1212 A gives no information as to the diameter of the hollow core and does not indicate any damping values. A process for drawing a thin fiber is also not indicated. Furthermore, no information as to the size of the fibers after the drawing process is given.
The production of photonic crystal fibers presents considerable problems since in a conventional drawing process, the preform of the fibers, in particular of the microstructured fibers, was very complicated because of the hollow spaces. At increasing temperature and relatively small structure sizes, the hollow spaces tend to collapse as a result of the surface tension.
The collapse of individual parts of the structure led to collapse of the total preform, so that the complete structure of the fibers collapsed in the most unfavorable case.
FR 2 606 866 has disclosed a process for producing fibers by means of a drawing process using two heating devices. In the process described in FR 2 606 866, heating is effected in two separate heating zones which are arranged in series. As materials which are heated by means of the device described in FR 2 606 866, mention is made of polymers, in particular PMMA.
The two-stage heating indicated in FR 2 606 866 prevents simultaneous heating of the outer part and the inner part of a preform. The resulting inhomogeneous heating when using the process and the apparatus described in FR 2 606 866 leads to collapse of the internal structure of the preform.
Further documents which relate to the production of glass fibers are U.S. Pat. No. 7,374,714 and US 2005/0274149, but with only one heating device.
DE 37 04 054 has disclosed a method of collapsing a glass tube. Since the glass tube firstly has to be collapsed, i.e. it must already be hot, before the CO2 radiation can bring about any effect at all, the CO2 laser radiation mentioned in DE 37 04 054 cannot be used for simultaneous heating by means of a plurality of heating devices.
Furthermore, in DE 37 04 054, the inner region of the glass tube is heated from the inside and by means of radiation (by means of CO2 laser), which cannot pass through the glass from the outside, through the glass tube which is open at the top.
Further documents on the subject of glass fibers and the production of fibers are DE 698 27 630 and U.S. Pat. No. 6,861,148.
DE 10 2011 103 686 A1 has disclosed a process for producing microstructured fibers, i.e. PCFs, in which microstructured fibers, very particularly preferably photonic crystal fibers, can successfully be drawn. According to DE 10 2011 103 686 A1, the disclosure content of which is fully incorporated by reference into the present patent application, the fiber material is produced by means of a drawing process from a preform of the fibers, in particular the microstructured fibers, and is heated by means of at least one first heating device and at least one second heating device to a drawing temperature, where the first heating device is a heating device which provides a temperature which is above the softening temperature of the fiber material. Preference is given to temperatures at which the viscosity is in the range η=104 dPas to 107.6 dPas. This results in the temperature preferably being from 10 K to 100 K above, in particular from 20 K to 60 K above, the softening temperature of the fiber material. The second heating device is, according to DE 10 2011 103 686 A1, an IR heating device as disclosed, for example, in WO 00/56674, the disclosure content of which is fully incorporated by reference into the present patent application, having a temperature of >1300 K, in particular >1500 K, in particular >2000 K, preferably >2500 K. First and second heating devices act essentially simultaneously or at the same time in a single heating zone. The preform of the fiber is then preferably heated from the outside in a single-stage heating operation.
The single-stage heating as per DE 10 2011 103 686 A1 in a heating zone having two heating devices achieves simultaneous heating of the outer and inner parts of the preform and prevents collapse of the internal structure.
The softening point of various fiber materials is defined at different viscosities, depending on the class of the fiber material. In general, the softening point is in the viscosity range from η=104 to 108 dPas.
In the case of glass materials and glass-ceramic materials, the softening point TEW is the temperature at which the glass has a viscosity of η=107.6 dPas in the ambient atmosphere. In this respect, reference may be made to “Schott-Guide to Glass”, by Heinz G. Pfänder, Chapman & Hall, 1996, page 21, in particular table 2.1, the disclosure content of which is fully incorporated by reference into the present patent application. As likewise shown in Heinz G. Pfänder “Schott-Guide to Glass”, Chapman & Hall, 1996, page 21 in FIG. 2.3, the viscosity versus temperature curve is different for different types of glass. Thus, different softening points or softening temperatures TEW are obtained for different types of glass. An example in DE 10 2011 103 686 A1 of a glass having low softening temperatures is, for example, a heavy flint glass, e.g. the glass SF6 from SCHOTT AG, Mainz, having a softening temperature of TEW=519° C. (n=107.6 dPas).
As a result of the use of two heating devices, the process described in DE 10 2011 103 686 A1 prevents uncontrolled changes in the hollow structure from occurring.
US 2005/0025965 A1 has disclosed an optical glass fiber having a hollow core composed of a chalcogenide glass. The glass fiber known from US 2005/0025965 A1 is a glass fiber having a photonic band gap. US 2005/0025965 A1 has not disclosed any arrangement of hollow bodies which leads to damping values of less than 2 dB/m, preferably less than 1 dB/m, in particular less than 0.3 dB/m. US 2005/0025965 A1 likewise does not disclose that the crystal fiber is a mode-containing crystal fiber.
US 2012/0141080 A1 likewise discloses a glass fiber having a photonic band gap (photonic gap) and a hollow core composed of a chalcogenide glass. As in US 2005/0025965 A1, US 2012/0141080 A1 does not say anything about arrangements of the hollow tubes which lead to low damping values or mode maintenance during transmission.
US 2008/0199135 A1 describes a glass fiber in which cladding, which can be made of a chalcogenide glass, surrounds a light-conducting core. The cladding itself has a plurality of holes which are arranged in hexagonal cells which in turn form an Archimedes lattice. As in the abovementioned documents, there is no information as to the arrangement of the hollow tubes around the core, which leads to low damping values.
It is an object of the invention to overcome the disadvantages of the prior art and provide an optical fiber which allows transmission of electromagnetic radiation in the IR wavelength range with low losses. Furthermore, a process for producing such a fiber should be provided.